If chips can be packaged more densely on the surface of a circuit board, the dimensions and cost of the module can be reduced and system performance improved. One possible method of maximizing packaging densities involves placing chips atop one another to form three-dimensional stacks referred to as stacked-chip devices or stacked-die devices. Over the past several years there has been great interest in stacking chips where possible. Such chip-stacking schemes include stacking a number of decreasing sized chips in order to facilitate the wire-bonds or stacking a number of same-sized chips using spacers or employing a beveling technique. Typically the lower-most die of the stacked-die device is attached to the substrate using a die-attach material such as a paste-based adhesive. The same die-attach material is used to attach subsequent die in the stacked-die device one to another. Typically the paste-based adhesive is applied as a liquid, the die is placed on the substrate (or upon another die), and the adhesive is then cured. During curing, the adhesive is raised to a relatively high temperature. To attach a subsequent die of a stacked-die device, the process is repeated. As the trend moves toward stacking more die, from 2–4 stacked die, in typical devices today, to 6–8 stacked die in the near future, and more, problems arise with the die-attach material. Repeatedly raising the temperature to cure each subsequent adhesive layer causes degradation of the previously applied adhesive layers.
Moreover, in some situations, the use of a paste-based adhesive as a die-attach material is not optimal, and is being replaced by a film die-attach material. For example, for some applications an extremely thin die may be desired. A typical die may be 725 microns thick, but for a given application (e.g., wireless communications), a die 25 microns thick may be desired. For such thin die, the imbalance in metal density from one side of the die to the other causes the die to warp. Such warping renders the use of a paste-based adhesive as a die-attach material problematic as the warped die does not maintain contact with the paste-based adhesive throughout the curing process.
Additionally, where the die package has dimensions approximately equal to the die, using a paste-based adhesive for a die attach material may lead to bleedout of the paste, which may interfere with a subsequent wire-bonding process.
These problems are addressed through use of a lamination process using a film die-attach material such as a thermoplastic-based film, which is beginning to gain popularity as the die-attach material of choice in stacked chipscale packages. Such material have many desired characteristics including good flowability and adhesive/cohesive strength.
In a typical lamination process using a thermoplastic film for the die-attach material, the die is laminated to the substrate (or to another die) under high temperature and pressure. The film has enough adhesive strength to hold the die flat (prevent warping) during the lamination process. To attach a subsequent die in order to form a stacked-die device, the process is repeated. This means that the die-attach film of a previously attached die is subjected to the heat and pressure of repeated laminations processes.
The repeated thermal processing can cause voids and stress in the die attach film causing delamination and warping. As a result, extreme care must be taken during successive lamination processes to prevent previous die attach film layers from delaminating or experiencing excessive deformation. This limits the ability to achieve void-free bond lines across the several dies of a stacked-die device. Moreover, the voids, cracks, and other defects caused by repeated high-temperature/high-pressure processes leads to a reduction in performance reliability.